Isolated power converters are widely used in various kinds of off-line power supply systems. An isolated power converter typically includes a primary-side circuit and a secondary-side circuit, which outputs a substantially constant voltage or current in the secondary-side circuit by controlling a power switch in the primary-side circuit. In order to avoid damage on the circuit due to over voltage conditions, it can be determined if an output voltage is in an over voltage condition according to feedback values, and that can be used to protect the isolated power converter in such a case. Some isolated converters obtain feedback values by employing an auxiliary winding in the transformer.